The present invention relates to a rotary apparatus for plasma immersion-assisted treatment, in particular ion implantation treatment, of three-dimensionally shaped workpieces, of which at least one is immersed, within a vacuum chamber, at least temporarily into an ionized plasma while it is exposed to a periodically pulsed high voltage, so that the extracted ions not only react on the workpiece surface but are implanted beneath the workpiece surface; and a treatment method using this rotary apparatus.
In many industrial sectors today, surface properties of properties are already being deliberately modified by depositing or converting thin films under vacuum. Also known, in addition to deposition methodsxe2x80x94which apply or deposit atoms or molecules onto a product surface from the gas phase, or by atomization or evaporation using a heating system, or by electron beam or arcxe2x80x94are non-deposition methods such as thermal and plasma treatment in inert or reactive gases, electron and laser treatments, or the implantation of inert, reactive, nonmetallic, or metallic ions into a substrate material.
Methods and apparatuses which combine an ion or plasma treatment with a coating operation are already known. These methods constitute the present standard for low-temperature deposition of high-melting-point alloy films which are used, for example, to protect mechanical components, e.g. highly stressed metal components, from wear, or to modify optical components, for example camera optics and spectacle lenses. Conversion of the applied ion energy into heat is utilized in this context. The ions are generated either in ion beam sources with or without mass separation, or in plasmas that either are excited in additional plasma sources with direct current, high-frequency currents or with microwaves, or are already present in any case in the context of plasma-assisted coating processes.
In the above described ion beam sources, ions are preaccelerated and directed onto one side of the workpieces to be treated. What is achieved thereby is a treatment or coating of substantially flat substrate surfaces. With the plasma technologies described above, however, negative DC electrical voltages are applied to the workpieces; these voltages can range from approx. 50 V to 1 kV, and with them the ions are directed, from all sides and in undirected fashion, not only onto substantially flat components but also onto the surface of three-dimensionally shaped components. At low ion energies (less than 1 keV), implantation effects such as doping, nonthermal alloying of nonequilibrium phases, interface mixing for adhesion enhancement, or accelerated diffusion via ion beam-induced structural defects are improbable and thus not industrially usable. For reasons of profitability, however, conventional ion-beam treatment (which is suitable for the purpose) is used in cost-sensitive production sectors (such as machine construction) only for specific and very limited product sectors.
A cost-effective plasma immersion-assisted ion implantation technology/has been developed by J. Conrad, J. L. Radke, R. A. Dodd, and F. J. Worzala and described in J. Applied Physics 62 (1987), p. 4591 (see also U.S. Pat. No. 4,764,394 issued to J. Conrad). In this so-called plasma immersion ion implantation method, also called PII, the workpieces are immersed, as with ion- or plasma-assisted coating, in a plasma. In contrast to the latter method, the magnitude of the applied negative voltage is from several kV to a few hundred kV, as a result of which the extracted ions not only react on the workpiece surface but are implanted beneath it, and initiate the aforementioned implantation effects. The high voltage is applied in the form of short pulses with a length of between several xcexcs and several hundred xcexcs, at a repetition frequency of between several hundred Hz and several kHz, in order to control heat input and at the same time minimize the complexity of the high voltage power supply. A further advantage of this pulsed technique is that buildup of the cathodic plasma edge layer along the workpiece contour, in which the voltage drop essential for ion acceleration occurs, is continuously being restarted. Immediately after the pulse start, an ion bombardment directed in perpendicular fashion onto the particular sample surface is thus always guaranteed, even in depressions or on elevations of structured geometries. Insufficient or raking ion bombardment, which occurs in unfavorable fashion in ion beam treatments, occurs with PII at the end of very long pulses when the plasma edge layer has expanded sufficiently far away from the plasma surface to create an enveloping plasma front that no longer follows the sample contour everywhere. This plasma edge layer propagates very quickly, e.g. within a few microseconds.
The PII processes that have hitherto been disclosed are used only for ion implantation, for example for nitriding steels or doping semiconductors. In this context, usually a separate plasma is generated by autonomous DC discharge or by high-frequency or microwave excitation, and the reactive components are usually admitted into it in gaseous form.
U.S. Pat. No. 4,764,394 also describes, for carrying out the PII treatment method, an apparatus that substantially has a high-vacuum treatment chamber with electrically conductive walls, for example made of stainless steel. All the conductive walls of this chamber are electrically connected to one another and to ground.
A three-dimensionally shaped workpiece is placed on an electrically conductive stationary pedestal spaced away from the chamber walls, and the conductive pedestal is joined to an electrically conductive support arm which holds the workpiece immovably and in electrical contact on the pedestal. The conductive support arm is electrically insulated from the conductive walls of the chamber by an insulator. High-voltage pulses are delivered to the workpiece from an external high-voltage pulse generator via a high-voltage line that is connected to the support arm and the pedestal.
When a high vacuum has been produced in the chamber, gas is admitted into the chamber to form a plasma that surrounds the workpiece (immersion). This gas is a mixed gas that contains the components necessary for implantation treatment. This neutral gas mixture within the chamber is ionized, for example, by a diffuse electron beam that proceeds from a heating coil inside the chamber.
U.S. Pat. No. 4,764,394 also mentions various other sources for the gas mixture needed for the PII treatment, e.g. by evaporation of liquids and solids. By way of magnet elements placed outside the chamber, there is generated inside the chamber a magnetic field that deflects the diffuse electrodes proceeding from the electron source away from the chamber walls and into the interior of the chamber, where they can collide with gas atoms or molecules and ionize the gas.
The above US Patent mentions neither a coating treatment of workpieces nor the use of a large-area coating plasma as an immersion plasma. In addition, the apparatus described in this US Patent does not provide for rotation of the workpiece being treated within the reaction chamber.
Since it is necessary, in the case of the aforementioned plasma immersion-assisted treatments, for high-voltage pulses of up to 100 kV to be delivered to the workpiece or workpieces within the high-vacuum treatment chamber, movement o f the workpiece by way of a rotary drive is possible only with special designs. This is complicated, among other factors, by the risk of high-voltage flashovers; there is a risk in particular of linear creepage sparks, i.e. discharges along regions in which metal, insulating ceramic, and plasma meet one another. In addition, reliable contact must be made to the moving parts, since only durable planar contacts make possible nonsparking delivery of voltage or current.
The basic idea of plasma immersion-assisted ion implantation is that a workpiece is immersed in a plasma and then, by application of a pulsed high voltage, experiences homogeneous treatment on all sides by ion implantation. The use of a magnetron sputtered plasma as the immersion plasma allows this ion implantation method to be combined with coating techniques.
Plasma sources, but principally coating plasma sources, often have a certain directional characteristic. In order to obtain homogeneous, uniform treatment of the workpiece even in this situation, it must be moved during the process. In known coating methods, this is achieved by way of a rotary drive. Because of the pulsed high voltage that is applied to the workpieces in the PII method, however, this has hitherto not been possible for that PII process.
In view of the above, one object of the present invention is to provide a rotary drive for plasma immersion-assisted treatment of workpieces such that with it, ion implantation operations and ion-assisted low-temperature coating operations can be performed economically, with high adhesion and structural quality, even on three-dimensionally structured workpieces of complex geometry; such that multiple workpieces can be rotated; and such that economical, homogeneous, and temperature-stable coating using the PII treatment method is achieved without the risk of high-voltage flashovers, linear creepage sparking, or poor high-voltage delivery.
An apparatus achieving the above-described object for rotating at least one workpiece, in particular for performing a plasma immersion-assisted treatment includes, among other things.
at least one rotatably driven sample holder in the form of a rotatably mounted, electrically conductive sample rod;
a rotary drive that engages on each sample rod in order to rotate it;
a high-voltage lead-in that delivers high voltage to the sample rod; and
at least first insulating means which insulate the sample rod (11) from the rotary drive in a manner withstanding high voltage.
The fundamental idea of the design of the rotary drive according to the present invention is that the entire apparatus is not at high-voltage potential, but rather that the actual driving and moving parts are at low-voltage potential, e.g. grounded, in a floating or defined state at up to the 1000 volt level. The insulating means (e.g., an arrangement) which are provided between the sample rods and the other driving and moving parts, and whose dimensions depend on the maximum voltage, electrically separate the sample rods, which serve to attach the workpieces, from the other parts of the apparatus. Contacting and delivery of high voltage to the sample rod or rods is accomplished at the other end thereof via planar wiper contacts.
Described below is an embodiment of the rotary drive usable for plasma immersion-assisted treatment in particular of three-dimensional workpieces, and exemplary embodiments of the method according to the present invention.